A typical Power-over-Ethernet (PoE) application involves supplying power from power-sourcing equipment (PSE) to a powered device (PD) through a cable (e.g., Category-3 cable, Category-5 cable, Category-6 cable, etc.). During operation, the PSE performs a discovery routine by testing for the presence of a known impedance on the PD end of the cable (e.g., a 25K-Ohm resistor). If the PSE determines that such an impedance is present, the PSE supplies power to the PD through the cable and periodically checks to make sure the PD is still present. If the PSE determines that the PD is no longer present, the PSE re-performs the discovery routine by continuing to periodically test for the presence of the known impedance on the PD.
When the PSE supplies power to the PD through the cable, the possibility exists that the temperature of the conductive material within the cable (e.g., copper) could increase beyond a safe limit. Once the temperature exceeds this limit, the PoE infrastructure and perhaps the surrounding environment could sustain permanent damage. In some cases, the results could be catastrophic (e.g., the cable insulation could melt, the cable could start a fire, a trace in a patch panel could fuse and become open, etc.).
For example, suppose that a PSE delivers power to a PD through wire pairs of a cable. Further suppose that there is a substantial increase in DC resistance through a wire in one of the wire pairs (e.g., the wire is suddenly cut, the wire suddenly provides high impedance due to faulty manufacturing, etc.). In response to a reduction in current through the failed wire of that wire pair, more current may pass through the non-failed wire of the wire pair. Under such a condition, the PSE may be able to detect the failure and automatically shut off current through that wire pair (e.g., the PHY of the PSE may sense impaired data delivery or loss of the link, a TDR circuit of the PHY may flag an open condition on that wire pair, etc.).
However, in some situations, the PSE may not be able to detect the increase in DC resistance and thus continue to supply current through the cable. For instance, the DC resistance could increase but not affect data flow (e.g., in a 10BaseT system). Additionally, due to the particular implementation of PHY circuitry and/or lower losses in the cable, the PSE may not be able to predict the potential overheating of a cable. Furthermore, the PSE may not even have a PHY/TDR attached to the cable such as when the PSE supplies power only and no data on two pairs in a cable (e.g., 10/100 Ethernet). Examples of situations where a PHY is absent include mid-span power systems, and switches providing power through unused pairs. Moreover, even with PHY-based TDR, there are limitations in performing accurate temperature measurements since the TDR typically has no direct access to the DC resistance of the cable that an inline power controller has.
To avoid creation of a catastrophic event due to cable overheating, some have considered employing an infrared detection circuit to monitor the temperature of the cable. In particular, an infrared camera of the infrared detection circuit would scan the cable, or a bundle of cables, in an attempt to identify whether the temperature of the cable ever surpasses a critical level. If the cable temperature were to exceed this level, the infrared detection circuit would then send a signal to the PSE directing the PSE to longer supply power to the PD through the cable.